High-level neural functions, including perception, cognition, and memory, rely on the coordinated activity of at least thousands of cortical neurons. The synchronized neuronal activity of large cortical cell assemblies results in electrical oscillations in electroencephalographic records and in local field potentials. Psychiatric, neurological, and neurodegenerative diseases, including schizophrenia, epilepsy, and Alzheimer's disease are characterized by a change in oscillatory timing and patterns. However, it is currently not known which therapeutic interventions could restore temporal control of neural circuits to achieve improvements in cognitive function. It has been suggested that the hippocampal theta rhythm provides temporal control for neuronal firing patterns that support memory processes, but this can only be tested if theta oscillations can be controlled with high temporal precision. We therefore propose to use the temporal precision of optogenetic inactivation techniques in neural circuits for theta generation to test the hypothesis that theta oscillations provide temporal coordination for hippocampal neurons during memory encoding and retrieval. We will test this hypothesis in two aims. The first aim is to attain precise tempora control of the theta rhythm and the second aim is to use temporally precise disruption to determine whether the coordination of hippocampal spike timing by theta oscillations is required during memory acquisition, retention, or retrieval. For both aims, we will infuse AAV9-Arch3.0-EYFP or AAV9-ArchT3.0-EYFP into medial septum and place recording electrodes in the hippocampus and/or the medial entorhinal cortex of rats. AAV9 was selected as a viral vector because it provides widespread infection of brain tissue, and Arch3.0 and ArchT3.0 were selected as opsins because they are effective light-induced inhibitors of neuronal activity. After recovery from surgery, rats will, for aim 1, be trained to continuously run on a track or randomly forage. Behaviorally relevant illumination protocols will be used to determine the effects of oscillating and discrete light pulses in the septal area on hippocampal theta oscillations. For all protocols, the analysis will focus on changes in the amplitude and frequency of theta oscillations and also on the effect of theta/gamma coupling. For aim 2, rats will be trained on a figure-eight delayed spatial alternation task with distinct encoding, retrieval, and retention phases. On separate days, light pulses will be delivered during one of these phases to determine when theta oscillations are required. For light- stimulation protocols that disrupt behavioral performance, we will examine the changes in the spike timing of hippocampal cells during decreased memory performance. By using temporally precise inactivation of a pacemaker for oscillations, the proposed aims will determine which phases of a memory task require oscillatory neural activity and to what extent the precisely timed neural activity in hippocampus is required for memory processes. Because electrical stimulation paradigms can also be effective by inhibiting ongoing neuronal activity, these experiments will provide new insight for the therapeutic use of deep brain stimulation.